Adaptive Emergency Message Broadcast Based on Network Connectivity States for Vehicular Ad Hoc Networks in Highway Environments

Broadcast plays a significant role in the emergency message propagation in Vehicular Ad hoc Networks (VANETs). However, current broadcast relay strategies easily cause serious message loss and larger broadcast costs due to the poor environmental adaptability. In this paper, to handle the above problems, we propose an Adaptive Connectivity-Aware Relay (ACAR) strategy, which has two striking features: multiple network connectivity states and dynamic broadcast relay policies. We design four network connectivity states and their corresponding four broadcast relay policies. In disconnected networks, only the vehicle with the same movement direction as the message propagation direction acts as the relay, so as to relieve the message loss. Moreover, different broadcast periods are allocated for different applications, so as to reduce the broadcast costs. In connected networks, the link quality and transmission distance are adopted to select the relay in the sender-based relay pattern, so as to address the relay invalidation problem. Further, different candidate relays are assigned different broadcast priorities and different broadcast waiting time in the receiver-based relay pattern, so as to address the transmission conflict problem. Simulation results show that ACAR outperforms existing competing schemes in terms of end-to-end delay, broadcast success rate, and packet overhead.